In a time before The Dark Knight, before the Marvel Cinematic Universe, M. Night Shyamalan’s Unbreakable felt like something special. This was a genre film – a superhero film, even if its marketing materials carefully disguised that fact – that took itself seriously. In retrospect, perhaps a touch too seriously; for all its visual panache, the film’s flaws – its dryness, its pacing, that coda – can’t be easily ignored. As a teenager, though, seeing a genre that had recently had Batman & Robin as its touchstone find real gravitas felt revolutionary.

It makes sense that Shyamalan would return to this story, particularly given the subsequent dominance of the superhero genre. As much as I hated Split – more due to the way it exploited and glorified mental illness than any judgment on its filmic quality – its post-credits incorporation into the Unbreakable was less surprising than …sensible. Inevitable. Glass kicks off about where you’d expect from that reveal, returning to David Dunn (Bruce Willis), now a part-time superhero assisted by his adult son (Spencer Treat Clark), hunting down and confronting the superhuman ‘Horde’ (James McAvoy), aka Kevin Wendell Crumb.

A standard superhero showdown isn’t what Shyamalan has in mind, though. It’s about the fifteen minute mark of Glass when Willis and McAvoy are implausibly interrupted by a crowd of police officers at the beck and call of psychiatrist Dr Ellie Staple (Sarah Paulson). They’re taken into custody, imprisoned with carefully-designed cells: Kevin is surrounded by ‘hypnosis’ floodlights that trigger a change in personality, while David’s cell can be flooded with water – if you recall from Unbreakable, his kryptonite – at the snap of Ellie’s fingers.

So far so good. This premise cleverly allays the conflict between our superhero and supervillain, while also, eventually and, again, implausibly, allowing for the incorporation of “Mr Glass”, Samuel L. Jackson’s villain from Unbreakable (here explicitly presented as possessing mental superpowers) into the narrative. It also gives the film a compelling hook. Rather than trying to beat the big budget superhero flicks at their own game, Shyamalan instead positions himself to examine the psychology of superheroes (something the films of a decade prior – Hancock, Kick-Ass, Super – tried and mostly failed to achieve).

Two problems with this. I’ve already alluded to the implausibilities inherent in this story twice, but oh boy they keep coming. The big problem is that Paulson’s psychiatrist isn’t interested in understanding her superpowered captives, but resolutely convincing them that they’re deranged. As an audience, you don’t buy this for one second; if she were convinced they were basketcases, why the elaborate water trap in David’s cell? This isn’t necessarily narratively inconsistent – though you have to wait ‘til the third act to find out why (what a twist!) – but it does immediately undercut anything engaging about Glass’s premise.

The second problem is a more fundamental one. Shyamalan simply doesn’t have anything interesting to say about the pathology of superheroes. He disguises this with portentous (but vacuous) dialogue and insufferably meta-references to genre tropes, but he’s more interested in establishing his own superhero universe to have anything interesting to say about the most over-saturated market in modern cinema. If Glass was content to be silly, I could forgive this, but the assumption of arthouse gravitas fits as well as worn-out spandex.

There’s a theme running underneath all this – a theme of acceptance. A theme that doesn’t really become clear until the closing minutes. It makes sense, though. Both Unbreakable and Split were, at their core, films about acceptance: acceptance of yourself, your flaws and foibles and your strengths. But where Split muddied its message with its clumsy, arguably offensive presentation of mental illness, Glass just can’t manage to find anything new to say, any way to elevate a message that’s obviously of great importance to Shyamalan.

While Glass isn’t a total trainwreck, it’s especially disappointing because it’s a reminder of what Shyamalan can do so well. While the film’s obligatory action scenes fall flat, Shyamalan’s confidence with horror/thriller grammar sings out in the film’s suspenseful midsection. He remains an actor’s director too; Willis, who’s been phoning it in for a while now, doesn’t have a lot to do here, but carries himself with the kind of presence that made him a movie star in the first place. It’s just a shame that Glass’s writing shatters its successful elements.